In a radio communication system for mobile communications of a mobile telephone, etc., various multiplex techniques are used for efficiency of signal transmission in limited resources. In the cellular system for mobile communications, as for an uplink (UL) of communications in an uplink direction from a user terminal to a base station, code multiplexing of signals from a plurality of user terminals to a base station in an uplink control channel for transmitting control information is considered. In an LTE (Long Term Evolution) system considered in 3GP (3rd Generation Partnership Project) of an international standardization group of mobile telephones, an uplink control channel is called PUCCH (Physical Uplink Control Channel) and is stipulated as appropriate in the system resources. In this PUCCH, to share a specific frequency resource among a plurality of user terminals, a configuration of code multiplexing of signals from a plurality of user terminals is adopted for the frequency resource. As a code sequence used for code multiplex, diffusion at two stages of a CAZAC sequence and an orthogonal sequence is used. Signals transmitted in the PUCCH include a demodulation reference signal (DM-RS), control information, etc. The control information includes an Ack/Nack signal for reporting Ack (Acknowledgement) or Nack (Negative Acknowledgement) indicating the demodulation result of a reception signal in a user terminal in a downlink of communications in a downlink direction from a base station to the user terminal and the like.
FIG. 13 shows an example of a code sequence used for each user terminal when code multiplex is performed in an uplink control channel. FIG. 13 shows resource allocation in two code sequences two-dimensionally as a assignment example of code sequence when applied to Ack/Nack signal resources (which will be hereinafter called Ack/Nack resources). In FIG. 13, the horizontal direction indicates the cyclic-shifted amounts of a cyclic-shifted sequence according to a CAZAC sequence and a code sequence having twelve cyclic-shifted amounts of 0 to 11. The vertical direction indicates a plurality of orthogonal codes as an orthogonal sequence and four-symbol codes (1, 1, 1, 1), (1, −1, 1, −1), (1, −1, −1, 1) . . . are used for the orthogonal codes. Each of ACK#0, ACK#1, . . . is the number of the Ack/Nack resource and indicates the position of the resource allocated to each user terminal. The Ack/Nack resources of each user terminal are used at intervals of two cyclic-shifted amounts in the same orthogonal code and are used at interval of one cyclic-shifted amount between the adjacent orthogonal codes, whereby allocation to the adjacent resources is not conducted at intervals of one allocated resource and inter-symbol interference is circumvented (for example, refer to Non-patent Literature 1). According to such allocation of the code sequence, inter-symbol interference is circumvented, and it is made possible to multiplex signals of a plurality of user terminals, so that the effect of increasing the system capacity can be provided.
To improve reliability in a single user terminal in an uplink control channel, application of transmission diversity for transmitting a signal using a plurality of antennas is assumed. In this case, as a simple solution according to the related art to realize the transmission diversity, application of CDD (Cyclic Delay Diversity) for transmitting a cyclically shifted signal in the same code sequence from a plurality of antennas is considered (for example, refer to Non-patent Literature 2).
Delay diversity is a technique of transmitting the same signal from a transmitter by a plurality of antennas and controlling a delay amount so as to produce a sufficient time difference (delay) in signal between antennas. Accordingly, if spacing of the antennas is small, a sufficient difference (time difference) occurs in radio signals arriving at a receiver from the antennas of the transmitter. Thus, the receiver recognizes the difference in radio wave propagation path and can separate each signal for each path and extract the object signal. Accordingly, the diversity effect is provided. In the CDD, the delay time (phase on a frequency axis) is changed cyclically.
FIG. 14 shows an example of resource allocation when CDD is applied in a code sequence to perform code multiplex as shown in FIG. 13. In FIG. 14, Ant#0, Ant#1 indicates the position (cyclic-shifted sequence) of the resource allocated to each antenna as for the same Ack/Nack resource (here, ACK#0). As shown in FIG. 14, different cyclic-shifted amounts are used for signals transmitted from a plurality of antennas, whereby a phase-shifted amount (delay amount) is given and CDD can be realized. Such CDD is applied, whereby frequency selectivity is enhanced about signals from the user terminal and the diversity effect of space-frequency can be provided.